Color image projection systems that use liquid crystal (LC) microdisplay panels to modulate light projected onto a screen such as used in large-screen televisions face a number of technical challenges as well as manufacturing cost challenges. These challenges may be further explained with respect to a conventional LC-microdisplay-panel-based color image projection system. In such a system, there must be a light source that is then modulated by a light valve containing one or more LC microdisplay panel(s) before the resulting modulated-light is projected onto a screen for viewing by a user. To achieve the desired viewable color image, three primary colors (typically red (R), green (G), and blue (B)) are separated from the white light provided by the light source. These primary colors are separately modulated by one or more light valves and then recombined or superimposed to form the image.
Because the image is formed from the separately-modulated beams of RGB colored light, there must be some means of separating these colors from the white light provided by the light source. These color separation means include dichroic mirrors, prism cubes, and color wheels. The light valve may contain three LC microdisplay panels: one to modulate the red light, one to modulate the green light, and one to modulate the blue light. Alternatively, the light valve may have just a single LC microdisplay panel that simultaneously modulates the RGB light using subpixels (one subpixel for each color being modulated). More recently, Philips has proposed a single LCOS (liquid crystal on silicon) microdisplay panel projection system using a scheme called “scrolling colors”—that is, after RGB color separation, three rotating prisms are employed to “scroll” RGB colors in field sequence respectively for the red, green and blue beams. The scrolling RGB beams are then realigned and modulated by a single microdisplay panel before projected onto the screen.
Regardless of whether the light valve contains one or three LC microdisplay panels, a number of problems arise in the design of such conventional color projection systems. For example, the use of three dichroic mirrors increases component cost and introduces the problem of realigning the separated RGB signals. Any misalignment will blur and/or introduce color shifts on the projected images. Alternatively, if color wheels are used to separate the RGB light, substantial power losses are introduced, inhibiting effective use of the light source. Moreover, should three separate LC microdisplay panels be used to individually modulate the separated RGB light beams, expensive and cumbersome alignment lenses are necessary to realign the separately-modulated light beams into a single RGB image, adding to the expense of providing three LC microdisplay panels. Furthermore, if just a single LC microdisplay panel containing RGB subpixels is used to simultaneously modulate the red, green, and blue light beams, expensive and cumbersome alignment lenses are still necessary to direct the beams exactly to the respective RGB subpixels. In addition, a single LC microdisplay panel with each pixel containing R,G, and B colored subpixels requires a complicated design and manufacturing process which reduces the overall product yield and increases cost. The more advanced Philips single LCOS panel design still has the problem of realigning the separated RGB beams and the associated complicated optics and increased cost.
Accordingly, there is a need in the art for improved color image projection systems having simplified optics allowing a more efficient use of the light source without the necessity of realigning and recombining separate RGB colored images.